Re: qom device lifecycle interaction with hotplug/hotunplug ?

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 28 Nov 2019 17:57:56 +0000
Peter Maydell <peter.maydell@linaro.org> wrote:

> On Thu, 28 Nov 2019 at 17:27, Igor Mammedov <imammedo@redhat.com> wrote:
> >
> > On Thu, 28 Nov 2019 16:00:06 +0000
> > Peter Maydell <peter.maydell@linaro.org> wrote:  
> > > Once a device is hot-unplugged (and thus unrealized) is it valid
> > > for it to be re-hot-plugged, or is the assumption that it's then
> > > destroyed and a fresh device is created if the user wants to plug
> > > something in again later ? Put another way, is it valid for a qdev
> > > device to see state transitions realize -> unrealize -> realize ?  
> >
> > I don't think we do it currently (or maybe we do with failover but
> > I missed that train), but I don't see why it can't be done.  
> 
> Well, as Eduardo says, if we don't currently do it then
> we probably have a lot of subtly buggy code. Requiring it to work
> imposes a requirement on the 'unrealize' function that it
> doesn't just do required cleanup/resource releasing actions,
> but also returns the device back to exactly the state it was in
> after instance_init, so that 'realize' will work correctly.
> That's quite a lot of code auditing/effort if we don't actually
> have a current or future use for making this work, rather than
> just requiring that an unrealized device object is immediately
> finalized without possibility of resurrection.
> 
> If we do have a plausible usecase then I think we should document
> that unrealize needs to handle this, and also have a basic
> smoke test of the realize->unrealize->realize.
yep, if we talk about generic approach, it's a problem.
But if a concrete combo of device/hotplug controller is considered
where such flow were necessary, it should be possible.

But from the my very limited understanding, on real hardware,
once device is uplugged it's gone (finalized) from machine
perspective, so it's unclear to my why someone would use
realize->unrealize->realize hotplug scenario.

> 
> thanks
> -- PMM
>